Mountain Building
• Mountain building has occurred during
the recent geologic past
• American Cordillera—the western margin
of the Americas from Cape Horn to
Alaska
– Includes the Andes and Rocky Mountains
• Alpine–Himalaya chain
• Mountainous terrains of the western
Pacific
Mountain Building
• Older Paleozoic- and Precambrian-age
mountains
• Appalachians
• Urals in Russia
• Orogenesis is the processes that
collectively produces a mountain belt.
• Includes folding, thrust faulting,
metamorphism, and igneous activity
How do mountains form?
Mountain Building
• Several hypotheses have been proposed
for the formations of Earth’s mountain
belts
• None explain all observations as well as
plate tectonics
• Most – but not all – mountain building
occurs at convergent plate boundaries
Earth’s Major Mountain Belts
Creating a Mountain
Two mechanisms shown here
•Hot spot activity
•Subduction creating island arc
Convergence and Subducting Plates
• Major features of subduction zones
• Deep-ocean trench—a region where
subducting oceanic lithosphere bends and
descends into the asthenosphere
• Volcanic arc—built upon the overlying
plate
– Island arc if on the ocean floor or
– Continental volcanic arc if oceanic
lithosphere is subducted beneath a
continental block
The Aleutian Volcanic Island Arc
Subduction and Mountain Building
• Island arc mountain building
• Where two ocean plates converge and
one is subducted beneath the other
• Volcanic island arcs result from the
steady subduction of oceanic lithosphere
– Continued development can result in the
formation of mountainous topography
consisting of igneous and metamorphic rocks
Ocean–Ocean
Island arcs:
• Tectonic belts of high seismic
activity
• High heat flow arc of active
volcanoes (andesitic)
• Bordered by a submarine
trench
Ocean–Ocean
Subduction Zone
Island arc
Ocean–Continent
Continental arcs:
• Active volcanoes (andesite
to rhyolite)
• Often accompanied by
compression of upper crust
Ocean–Continent Convergent
Boundaries
Breadth of
arc-trench
gap
Breadth of
arc-trench
gap
Subduction and Mountain Building
• Andean-type mountain building
• Mountain building along continental
margins
• Involves the convergence of an oceanic
plate and a plate whose leading edge
contains continental crust
– Exemplified by the Andes Mountains
Ocean-Continent Subduction Zone
Continental volcanic arc
Subduction and Mountain Building
• Andean-type mountain building
• Building a continental volcanic arc
– Subduction and partial melting of mantle
rock generate primary magmas
– Magma is less dense than surrounding rock,
so it begins to buoyantly rise
– Differentiation of magma produces andesitic
volcanism dominated by pyroclastics and
lavas
Subduction and Mountain Building
• Andean-type mountain building
• Emplacement of plutons
– Thick continental crust impedes the ascent
of magma
– A large percentage of the magma never
reaches the surface and is emplaced as
plutons
– Uplift and erosion exposes these massive
structures called batholiths (i.e., the Sierra
Nevada in California and the Peruvian
Andes)
– Batholiths are typically intermediate to
felsic compositions
Andean-Type Plate Margin
Subduction and Mountain Building
• Andean-type mountain building
• Development of an accretionary wedge
– An accretionary wedge is a chaotic
accumulation of deformed and thrustfaulted sediments and scraps of oceanic
crust
– Prolonged subduction may thicken an
accretionary wedge enough so that it
protrudes above sea level
– Descending sediments are metamorphosed
into a suite of high-pressure, lowtemperature minerals
Subduction and Mountain Building
• Andean-type mountain building
• Forearc basin
– The growing accretionary wedge acts as a
barrier to sediment movement from the arc
to the trench
– This region of relatively undeformed layers
of sediment and sedimentary rock is called a
forearc basin
Parts of an Ocean–Ocean
Convergent Plate Boundary
Subduction and Mountain Building
• The Sierra Nevada and the Coast Ranges
• One of the best examples of an active
Andean-type orogenic belt
• Subduction of the Pacific basin under the
western edge of the North American
plate
• The Sierra Nevada batholith is a remnant
of a portion of the continental volcanic
arc
• The Franciscan Formation of California’s
Coast Ranges constitutes the
accretionary wedge
Mountains and Landforms of the
Western United States
Convergence and Subducting Plates
• Major features of subduction zones
• The forearc region is the area between
the trench and the volcanic arc
• The backarc region is located on the side
of the volcanic arc opposite the trench
Convergence and Subducting Plates
• Dynamics at subduction zones
• Extension and backarc spreading
– As the subducting plate sinks, it creates a
flow in the asthenosphere that pulls the
upper plate toward the trench
– Tension and thinning may produce a backarc
basin
Continental Collisions
• Two lithospheric plates, both carrying
continental crust
• Continental collisions result in the
development of compressional mountains
that are characterized by shortened
and thickened crust
• Most compressional mountains exhibit a
region of intense folding and thrust
faulting called a fold-and-thrust belt
• The zone where the two continents
collide is called the suture
Major Features of a
Compressional Mountain Belt
Terranes
• Geologically
distinctive regions of
Earth’s crust, each of
which has behaved as
a coherent crustal
block
Terranes and Mountain Building
• Another mechanism of orogenesis
• The nature of terranes
• Small crustal fragments collide and
merge with continental margins
• Accreted crustal blocks are called
terranes (any crustal fragments whose
geologic history is distinct from that of
the adjoining terranes)
Approaching Arc or Microcontinent
Collision
Accreted Microplate Terrane
Terranes and Mountain Building
• The nature of terranes
• Prior to accretion, some of the
fragments may have been
microcontinents
• Others may have been island arcs,
submerged crustal fragments, extinct
volcanic islands, or submerged oceanic
plateaus
Terranes and Mountain Building
• Accretion and orogenesis
• As oceanic plates move, they carry
embedded oceanic plateaus, island arcs,
and microcontinents to Andean-type
subduction zones
• Thick oceanic plates carrying oceanic
plateaus or “lighter” igneous rocks of
island arcs may be too buoyant to
subduct
Terranes and Mountain Building
• Accretion and orogenesis
• Collision of the fragments with the
continental margin deforms both blocks,
adding to the zone of deformation and to
the thickness of the continental margin
• Many of the terranes found in the North
American Cordillera were once scattered
throughout the eastern Pacific
Microplate terranes
added to western
North America over
the past 200 million
years
Continental Collisions
• The Appalachian Mountains
• Formed long ago and substantially
lowered by erosion
• Resulted from a collision among North
America, Europe, and northern Africa
• Final orogeny occurred about 250 million
to 300 million years ago
Appalachian
Mountains
Fault-Block Mountains
• Continental rifting can produce uplift
and the formation of mountains known
as fault-block mountains
• Fault-block mountains are bounded by
high-angle normal faults that flatten
with depth
• Examples include parts of the Sierra
Nevada of California and the Grand
Tetons of Wyoming
The Teton Range in Wyoming
Are Fault-Block Mountains
Fault-Block Mountains
• Basin and Range Province
• One of the largest regions of faultblock mountains on Earth
• Tilting of these faulted structures has
produced nearly parallel mountain ranges
that average 80 kilometers in length
• Extension beginning 20 million years ago
has stretched the crust twice its
original width
Fault-Block Mountains
• Basin and Range Province
• High heat flow and several episodes of
volcanism provide evidence that mantle
upwelling caused doming of the crust
and subsequent extension
The Basin and Range Province
Vertical Movements of the Crust
• Isostasy
• Less dense crust floats on top of the
denser and deformable rocks of the
mantle
• Concept of floating crust in
gravitational balance is called isostasy
• If weight is added or removed from the
crust, isostatic adjustment will take
place as the crust subsides or rebounds
The Principle of Isostasy
Vertical Movements of the Crust
• Vertical motions and mantle convection
• Buoyancy of hot rising mantle material
accounts for broad upwarping in the
overlying lithosphere
• Uplifting whole continents
– Southern Africa
• Crustal rebound
– Regions once covered by ice during the last
Ice Age
– Ice melts, crustal rebound and uplift
Uplift Formed by Removal
of Ice Sheet
Raised Beaches
Due to Isostatic Uplift
Continental Collisions
• The Himalayan Mountains
• Youthful mountains—collision began
about 45 million years ago
• India collided with Eurasian plate
• Similar but older collision occurred when
the European continent collided with the
Asian continent to produce the Ural
Mountains
Formation of the Himalayas
Tibet—not just mountains, a huge
plateau too
Continent–Continent
Convergent Boundary
Indian plate subducts
beneath Eurasian plate
60 million years ago
Indian subcontinent
collides with Tibet
40–60 million years ago
Accretionary wedge and forearc
deposits thrust northward onto Tibet
Approximately 40–20 million years ago
Main boundary fault develops
10–20 million years ago
India has collided with Asia
Exotic
terranes
Faults galore…
…and earthquakes
Focal mechanisms
Thrust
Strikeslip
Normal
Normal faults in the plateau
mean E-W extension
Himalayan collision ideas
A
complicated
explanation
emerges
The drooling lithosphere
So now we think we
have figured it out
Indian climate before
Himalayas
Monsoon Climate:
Tibet heats up and air rises
Moist Indian Ocean air sucked in
Clouds form as moist air blocked by
mountains